1. Field of the Invention
The present invention relates to a Ni based alloy flux cored wire which is used in welding of 9% Ni steel, various high Ni alloys, and the like.
2. Description of the Related Art
A welding material containing a Ni based alloy as a component is used in welding of, for example, a 9% Ni steel which is representative of low temperature service steels. The 9% Ni steel and the like are widely used for storage tanks and the like of, for example, LNG, liquid nitrogen, and liquid oxygen. In welding of the 9% Ni steel, in order to ensure the toughness, which is equivalent to that of the base material, of a weld zone at a cryogenic temperature of −196° C., a Ni based alloy welding material, rather than a welding wire containing a component similar to the 9% Ni steel having a ferrite microstructure (so-called matching composition wire), is used for a weld joint in general. This is because in the case where welding is performed by using the matching composition wire, the weld joint on an “as welded” basis may not ensure the strength and low-temperature toughness equivalent to those of the 9% Ni steel.
In recent years, in the case of specific welding materials, e.g., Ni based alloys, as well, gas-shielded metal arc welding by using a Ni based alloy flux cored wire has been expanded, where a higher operation efficiency is expected as compared with those in manual metal-arc welding and TIG welding. On the other hand, the Ni based alloy has a complete austenite microstructure and high hot cracking susceptibility and, therefore, it is difficult to ensure the compatibility between the hot cracking resistance and the welding operability, so that there are limitations to the welding position and the welding condition applicability. Then, as for the Ni based alloy flux cored wire, various studies have been made to improve the hot cracking resistance for the purpose of expansion of this applicability.
For example, in Japanese Unexamined Patent Application Publication No. 2005-59077 and Japanese Unexamined Patent Application Publication No. 2007-203350, reduction in the hot cracking susceptibility is intended by optimizing the wire components and deposit metal chemical components of the Ni based alloy flux cored wire, that is, by reducing contents of Cr and Nb which adversely affect the hot cracking resistance and ensuring deposit metal chemical components primarily composed of a Ni—Mo alloy system. For example, in Japanese Unexamined Patent Application Publication No. 60-46896, reduction of blowholes in the Ni based composite wire for welding of a low temperature service steel is intended by adding Ti, Al, Zr, and Mg to a flux, so as to facilitate deoxidation reaction of a molten metal and reducing a CO gas reaction.
As for each of the wires described in Japanese Unexamined Patent Application Publication Nos. 2005-59077 and 2007-203350, ensuring of the compatibility between an improvement in the hot cracking resistance and the all position welding operability is intended, but there is room for improvement in the porosity defect resistance (blowhole resistance).
Meanwhile, as for the wire described in Japanese Unexamined Patent Application Publication No. 60-46896, the deoxidizing components are added to the flux. Even when the deoxidizing components are added to the flux, most of the deoxidizing components in the flux are oxidized during welding before the deoxidation reaction of the molten metal is induced and are discharged as slag. Consequently, there is a problem that the effect of deoxidizing the molten metal is low. In this regard, it is necessary that large amounts of deoxidizing components be added to the flux to sufficiently obtain the effect of deoxidizing the molten metal, and there is a problem that the welding operability is degraded because of an increase in spatter by addition of large amounts of deoxidizing components. Consequently, development of the Ni based alloy flux cored wire has been desired, where the compatibility between the all position welding operability and the porosity defect resistance is ensured.